DE69811560T2 - vehicle tires - Google Patents

Description

The present invention relates to a vehicle tire improved in terms of rolling resistance and specific electrical conductivity, and in particular to an improved tread rubber with a smaller specific electrical resistance and a smaller hysteresis loss.

In recent years, from the point of view of environmental issues such as global warming, reducing the rolling resistance of pneumatic tires has become a major issue. Many attempts have been made, with tread rubber compounds reinforced with silica instead of conventional carbon black now considered advantageous.

Such mixtures have very low hysteresis losses, but the specific electrical resistance is very high. When using tires whose treads are made of such insulating rubber, the vehicle body becomes electrostatically charged, which causes problems such as vehicle fires, radio noise, interference with the lighting system and the like.

To solve this problem, the applicant has proposed a tire in Japanese Patent Application Publication No. JP-A-9-71112 in which the tread rubber is made mostly of a non-conductive rubber, but a conductive rubber compound is used in part to discharge static electricity, in which conductivity is induced by adding a conductive substance made of small solids such as carbon black or metal powder. It is better to to minimize the volume of the conductive rubber to keep the tread hysteresis loss small. In this case, however, the carbon black content must inevitably be increased to achieve the required conductivity. As a result, it is not easy to maintain the right balance between the conductive rubber and the rubber with low hysteresis loss in terms of hardness, wear resistance and the like. Furthermore, the higher hysteresis loss of the conductive rubber by adding carbon black may cancel out the improvement in rolling resistance.

It is therefore an object of the present invention to provide a tire whose rolling resistance and conductivity are improved.

According to the present invention, a tire includes a tread rubber whose radially outer surface forms the ground contact surface of the tire, the tread rubber at least partially comprising a conductive rubber extending from the radially inner surface of the tread rubber to the ground contact surface and composed of 100 parts by weight of diene rubber and 2 to 30 parts by weight of conductive short fibers formed by coating reinforcing short fibers with a conductive substance, the conductive rubber having a volume resistivity of at most 1 x 10⁸ Ω cm.

In conjunction with the accompanying drawings, embodiments of the present invention will now be described in more detail;

Fig. 1(A) is the chemical formula of a pyrrole chain;

Fig. 1(B) is the chemical formula of an aniline chain;

Fig. 2 is a cross-sectional view of an embodiment of the present invention;

Fig. 3 is a diagram explaining a measuring method for the electrical resistivity of a tire;

Fig. 4 is a cross-sectional view of another embodiment of the present invention;

Figs. 5-8 are enlarged cross-sectional views each showing an example of the conductive portion thereof;

Fig. 9 is a cross-sectional view of yet another embodiment of the present invention;

Fig. 10 is an enlarged cross-sectional view showing an example of the conductive portion thereof;

Fig. 11 is an enlarged cross-sectional view showing another example of the conductive portion;

Fig. 12 is a cross-sectional view of a cover tread and base tread assembly formed by an extruder.

In the drawings, the tires according to the present invention correspond to a pneumatic tire comprising a tread portion 2, a pair of axially spaced bead portions 4 each containing a bead core 5, a pair of sidewalls 3 extending between the tread edges and the bead portions 4, an annular carcass 6 extending between the bead portions 4 and a belt 7 arranged radially outside the carcass 6 and inside the tread rubber 9.

The carcass 6 comprises at least one cord ply extending between the bead portions 4 and turned up around the bead cores 5 to form a pair of turn-up portions and a main portion therebetween. As cords for the carcass, steel cords and cords made of organic fibers, e.g. polyester, nylon, rayon, aromatic polyamides and the like, can be used.

The belt 7 comprises the usual two cross layers or so-called breaker layers. Steel cords are preferably used as cords for the breaker belt. The belt can also contain a bandage layer, the cords of which essentially have a cord angle of 0º to the circumferential direction of the tire.

Each of the bead portions 4 is usually provided with a bead filler made of hard rubber, which tapers radially outward, between the turn-up portion of the carcass ply and the main portion.

The tread rubber 9 is arranged radially outside the belt 7 and forms the tread portion 2, wherein the tire tread or ground contact surface 2S is defined by the outer surface of the tread rubber in the radial direction.

According to the present invention, the tread rubber 9 comprises at least partially a conductive rubber compound 10. The conductive rubber compound 10 extends from the radially inner surface to the radially outer surface of the tread rubber 9 and forms at least partially the ground contact surface 2S of the tire.

The conductive rubber compound 10 consists of 100 parts by weight of a rubber base compound and 2 to 30 parts by weight of conductive short fibers mixed therein.

If the conductive short fibers exceed 30 parts by weight, the energy loss between them and the rubber increases sharply, while the rolling resistance does not decrease. Furthermore, the complex elastic modulus of the conductive rubber compound increases, while the wet performance decreases. Accordingly, the content of conductive short fibers is 30 parts by weight or less. If it drops below 2 parts by weight, it becomes impossible to achieve even the minimum required conductivity.

The conductive short fibers are created by coating short reinforcing fibers with a conductive substance.

As the short reinforcing fibers, there can be used: synthetic organic fibers such as nylon, rayon, vinylon, polyethylene, polystyrene, polyvinyl chloride, polyvinylidene chloride, aromatic polyamides, polyethylene terephthalate, polypropylene, cellulose and the like; vegetable fibers made from cellulose and the like such as wood pulp; and inorganic fibers such as glass, aluminum and the like.

In order to preserve the plasticity of the finished rubber, organic fibers such as nylon fibers and cellulose are preferably used. Nylon fibers are particularly preferred due to their high extensibility, elasticity and strength.

In view of the conductivity of the finished rubber, it is preferable to use fine fibers. However, if the fibers are too fine, they tend to felt, preventing the dispersion of the fibers in the mixture. Therefore, the diameter D is preferably in the range of 1 to 100 µm.

The length L of the short reinforcing fibers is preferably in the range of 10 to 6000 µm in view of the reinforcing effect. If the length is outside this range, the dispersion of the fibers in the mixture may be prevented and the desired effectiveness may be difficult to achieve.

The ratio L/D of the fiber length L to the fiber diameter D is preferably between 10 and 2000. If L/D is less than 10, the dispersion of the fibers becomes difficult. If it is greater than 2000, the short reinforcing fibers cause microscopic rubber cracks in the ground contact area of the tire and the wear resistance decreases. Furthermore, the number of short fibers in contact with each other increases excessively, resulting in an increased decrease in internal energy and an increase in rolling resistance.

As the conductive substance for coating the short reinforcing fibers, conductive polymers whose main chain has pi-electron conjugation, such as polypyrrole, polyaniline, alkylene oxide and the like, and metal salts can be used.

In view of the required adhesion of the short reinforcing fibers, conductive polymers are preferably used. Due to the stable conductivity, mixtures with a polypyrrole framework are preferred. or having a polyaniline skeleton. A blend having a polypyrrole skeleton as shown in Fig. 1(A) is a blend in which each polymer has a pyrrole chain 17 consisting of pyrrole rings 17A as a main chain. The blend having a polyaniline skeleton is a blend in which each polymer has an aniline chain 18 consisting of aniline rings 18A as a main chain as shown in Fig. 1(B).

When such conductive polymers are used, it is preferable to add a small amount of an electron-accepting substance such as iodine, arsenic (V) fluoride and the like or an electron-donating substance such as potassium, sodium and the like to increase the conductivity.

In connection with the coating process, a conductive polymer can be formed by polymerizing monomers in the presence of short reinforcing fibers.

To explain in more detail, when the conductive polymer is polypyrrole and the short reinforcing fibers are nylon, for example, it is formed by putting the fibers in an aqueous solution of ferric chloride (6) hydrate (FeCl₃-6H₂O), stirring the solution to disperse the fibers, adding an aqueous solution of pyrrole, stirring the solution for several hours to allow conjugated bonding, and taking out the fibers through a filter, washing them in water and methanol, and finally vacuum drying them. As a result, polypyrrole-coated nylon short fibers can be

On the other hand, if the conductive substance is a metal salt, various metallization processes such as electroplating and vacuum evaporation can be applied.

The amount of the conductive substance for the coating is set in the range of maximum 1 part by weight based on 100 parts by weight of short reinforcing fibers. The thickness of the coating is about 0.02 to about 0.1 mm.

The rubber base compound mentioned above contains as rubber diene rubber such as natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR) and the like or a combination of these.

If SBR is to be used, S-SBR is preferable. Furthermore, the use of S-SBR, whose glass transition temperature is -50ºC or less, is more preferred to reduce rolling resistance.

Furthermore, conventional additives such as rubber reinforcing agents, sulfur, anti-aging agents and the like can be added to the rubber base mixture.

Silica is preferably used as the rubber reinforcing agent because it has a low hysteresis loss, which contributes to reducing rolling resistance. The silica content may be 0 to 100 parts by weight, and preferably 0 to 70 parts by weight.

As silica, colloidal silica is preferably used because of the rubber reinforcing effect and the processing properties of the rubber, the relative nitrogen adsorption area (BET) of which is in the range of 150 to 250 m²/g and the dibutyl phthalate (DBP) oil absorption of which is at least about 180 ml/100 g.

In order to minimize the hysteresis loss, the conductive rubber compound 10 preferably does not contain any carbon black. However, the use of carbon black is possible. Furnace black (SAP, ISAF and HAF), acetylene black and boiler black can be used as carbon black.

The carbon black content is 0 to 45 parts by weight. If the carbon black content is more than 45 parts by weight, the hysteresis loss and rolling resistance increase. Furthermore, the electrical resistivity is completely reduced by the carbon, so it is not necessary to add short conductive fibers.

Instead of silica and carbon black, the short fibers can be used for rubber reinforcement. In this case, the hysteresis loss decreases further while a minimum rolling resistance is obtained. However, as explained above, it is preferable to limit the content of short fibers to 30 parts by weight or less.

Accordingly, the short conductive fibers contact each other, thereby imparting to the rubber compound 10 the good conductivity required for a tire. Furthermore, the required conductivity can be obtained by using a minimum amount of a conductive substance, which amount is greatly reduced compared to a method in which the conductive substance is directly added to the base rubber.

The sidewall rubber and the bead rubber constituting the sidewall portions 3 and the bead portions 4 are made of a conductive rubber compound having a volume resistivity of at most 1 x 10⁸ Ω cm. However, this conductivity is provided by carbon black according to a conventional method.

The specific electrical resistance of the tire between the tread and the rim should be less than 1 x 10⁻Ω cm. Furthermore, even after a running distance of 1000 km, it should preferably be maintained at less than 1 x 10⁻Ω cm.

Fig. 2 shows an embodiment of the present invention corresponding to a radial pneumatic tire for passenger cars.

The carcass 6 is composed of two cord plies arranged radially at an angle of 75º to 90º with respect to the tire equator C. Each ply is turned up from the axially inner side to the outer side of the tire around the bead cores 5 to form a pair of turn-up portions and a main portion therebetween.

The belt 7 is composed of two cross layers of parallel cords oriented at an angle of not more than 30º, in this example approximately 24º, with respect to the tire equator C.

In this embodiment, the tread rubber 9, which is arranged on the radially outer side of the belt 7, consists exclusively of the conductive rubber compound 10.

Comparison test 1

Various rubber compounds were prepared by changing the content of short conductive fibers, silica, carbon black and the like as shown in Table 1, and the various properties were measured. Furthermore, using these compounds Test tires were manufactured as tread rubber and the specific electrical resistance was measured.

The manufacturing process for the rubber compounds was as follows.

First, the diene rubber materials shown in Table 1 were mixed in a Banbury mixer at about 150°C for 4 minutes.

Then, this rubber mixture and 1.0 parts by weight of sulfur and 1.5 parts by weight of a vulcanization accelerator were further mixed at 80°C for about 4 minutes by biaxial pressure rollers.

This mixture was used as raw tread rubber to build a green tire and was also vulcanized at 170ºC for 10 minutes to prepare samples for measuring volume resistivity, wear resistance and loss coefficient.

Loss Factor Test: The loss factor or loss coefficient of each of the rubber compounds was measured using a viscoelastic spectrometer under the following conditions: dynamic deformation 2.0%, frequency 10 Hz, temperature 60ºC. Table 1 shows the reciprocals of the measured values using an index based on Ref. 1 equal to 100. The larger the value, the smaller, i.e. better, the loss coefficient.

Wear resistance test: The extent of wear of each rubber compound was measured using a Lanbone wear tester under the following conditions: surface rotation speed 50 m/min. load 1.5 kgf. slip rate 30% and 50% and sand discharge 15 g/min. Table 1 shows the average of two results at two Slip rates are indicated by an index based on Ref. 1 equal to 100. The larger the value, the higher the wear resistance.

Volume resistivity test: The volume resistivity was measured using a test piece of 15 cm x 15 cm x 2 mm under the following conditions: applied voltage 500 V, temperature 25ºC, humidity 50%. The results are shown in Table 1.

Tyre electrical resistivity test: To manufacture a 175/70R13 tyre, the green tread rubber was applied to a green tyre casing to form a green tyre, which was then vulcanised at 170ºC for 10 minutes. The tyre electrical resistivity was measured according to the German method WDK, Sheet 3. As shown in Fig. 3, the tyre 1 mounted on a standard rim R and filled to a pressure of 20 kPa was placed on a steel plate 31 which was galvanically insulated from a table 30. Thereafter, a load of 450 kg was applied to the tyre. Under these conditions, the electrical resistance between the rim R and the steel plate 31 was measured by means of an ohmmeter 32. The applied voltage was 500 V, the temperature was 25ºC and the humidity was 50%. Table 1

*) logarithmic indication

Conductive short fibers A:

Fibers: Nylon fibers, L = 800 um, D = 16 um

Conductive substance: polypyrrole resin

Conductive short fibers B:

Fibers: Nylon fibers, L = 400-600 um, D = 16 um

Conductive substance: polypyrrole resin

Comparison of Ref. 1 to 3 and Ex. 1 to 5 showed that the volume resistivity decreased with increasing content of conductive short fibers. However, when the content was less than 2 parts by weight, the rate of decrease of the resistivity was small. On the other hand, when the content exceeded 30 parts by weight, the rate of decrease of the wear resistance was undesirably large.

The comparison of Ex.3 and Ex.6 showed that even a change in the silica content did not lead to a decrease in the resistance.

The comparison of Ex.3 and Ex.7 showed that the resistance decreases more the more conductive short fibers are present.

The comparison of Ref.4 to 6 and Ex.8 to 9 showed that the specific electrical resistance is effectively reduced by the additional effect of the soot.

Fig. 4 shows another embodiment of the present invention. In this embodiment, the tire is a pneumatic radial tire for passenger cars. The carcass 6 and the belt 7 have the same structures as in the above-mentioned first embodiment. However, the tread rubber 9 is changed. The above-mentioned conductive rubber compound 10 is partially used.

The tread rubber 9 comprises a conductive portion made of the conductive rubber compound 10 and a main portion made of another rubber compound 11.

The main portion 11 is arranged on the radially outer side of the belt 7, which forms the largest part of the tread 2S. The conductive portion 10 extends from the radially outer surface of the belt 7 across the main portion 11 to the tread 2S, so that its radially outer end forms part of the tread 2S.

The rubber compound for main section 1 is a rubber that is designed to provide the best rolling resistance, wear resistance and Wet performance is given more weight than electrical conductivity. In this example, a non-conductive rubber is used which is composed of: 100 parts by weight of a rubber base, 30 to 100, preferably 40 to 70 and more preferably 40 to 60 parts by weight of silica and at most 30, preferably at most 10 and most preferably substantially 0 parts by weight of carbon black.

As the rubber base, diene rubber such as natural rubber (NR), styrene-butadiene rubber (SBR), isoprene rubber (IR), butadiene rubber (BR), acrylonitrile-butadiene rubber (NBR) and chloroprene rubber (CR) or a combination of these can be used. In particular, NR, IR, BR and SBR are preferably used.

Furthermore, various additives such as sulfur, vulcanizing agents, vulcanization accelerators, plasticizers, age inhibitors, silane coupling agents and the like can be added.

Bis(triethoxypropyl)tetrasulfide and alpha-mercaptpropyltrimethoxysilane can be used advantageously as silane coupling agents.

Therefore, the volume resistivity of this rubber can be more than 1 · 10⁻² Ω cm.

In addition, this compound can be used as a rubber base compound (mentioned above) for the conductive rubber compound 10.

Figs. 4 and 5 show an example of the conductive portion 10. In this example, the main portion of the conductive portion 10, which extends from the radially outer end to the vicinity of the inner end, has stretches, a substantially constant width Wt, however, the root portion located radially inside the main portion gradually widens toward the radially inner side to increase the contact area with the belt 7 and also prevent stress concentration. In its cross section, the contour of the root portion may partially describe a circle. The radial height H2 of the root portion is at most 20% of the total height H1 of the conductive portion 10, while the maximum width Wmax is about 1.2 to 5 times the width Wt of the main portion.

This conductive section 10 extends continuously in the circumferential direction of the tire and is arranged in the middle tire region, for example at the tire equator.

Grooves G are provided in the tread portion in the circumferential direction and at a distance from the conductive portion 10. However, axial grooves Y may cross the conductive portion 10. The continuous circumferential extension of the above-mentioned conductive portion 10 means that it is materially continuous in the lower tread portion. Furthermore, this conductive portion 10 may be formed at two or more axial positions.

Figs. 6, 7 and 8 show modifications of the cross-sectional shape of the conductive section 10.

In Fig. 6, the axial width gradually decreases from the inside to the outside.

In Fig. 7, the axial width gradually increases from the center both inwardly and outwardly, so that a narrow-width portion 13 is formed.

The minimum width Wmin is preferably in the range of 1.0 to 0.4 times the maximum width Wmax at the inner end.

In Fig. 8, the axial width gradually increases from the inside to the outside.

Furthermore, as shown by a dashed line in Fig. 7, the width may decrease inwardly and outwardly from a central wide portion 14.

The volume V1 of the conductive portion 10 is 2 to 20% of the total volume V0 of the tread rubber 9, whereby it is possible to provide a high-performance tire having good conductivity due to the conductive portion and having excellent wear resistance, rolling resistance and wet properties due to the main portion made of the silica base compound. If the V1/V0 ratio is over 20%, the wear resistance, rolling resistance and wet properties deteriorate. If it is under 2%, the specific electrical conductivity becomes insufficient.

Comparison test 2

In the same manner as explained above, test specimens and test tires of size 175/70R13 were prepared and the volume resistivity of the compound as well as the electrical resistivity, rolling resistance and wet properties of the tire were measured.

Volume resistivity test: The volume resistivity was measured as mentioned above. A value of 12 or less is preferable.

Rolling resistance test: The test tire was mounted on a standard R rim and inflated to a pressure of 200 kpa. The rolling resistance was measured at a speed of 80 km/h and a tire load of 345 kg using a tester. In Table 2, the results are indicated by an index based on Ref.1 equal to 100. The larger the index, the better the rolling resistance.

Wear resistance test: A passenger car equipped with test tires mounted on a standard R rim and inflated to 200 kpa was driven on expressways and highways for 30,000 km and the depth of the tread grooves was measured. The results are indicated by an index based on Ref.1 equal to 100. The larger the index, the better the wear resistance.

Wet Preferential Value Test: The test car was driven in circles on a wet slab track with a very low coefficient of friction, measuring the critical cornering speed. In Table 2, the results are given by an index based on Ref.1 equal to 100. The larger the index, the better the wet properties.

Tyre resistivity test: The test tyre's resistivity was measured as described above. A value of 8 or less is preferable. Table 2

*1) logarithmic indication

Conductive short fibers A:

Fibers: Nylon fibers, L = 50 um, D = 16 um

Conductive substance: polypyrrole resin

Conductive short fibers B:

Fibres: Pulp

Conductive substance: polyaniline resin

Table 2 shows that when the conductive rubber compound contains 2 to 30 parts by weight of conductive short fibers, the specific electrical conductivity, rolling resistance, wear resistance and wet properties can be adjusted in a well-balanced manner to a high level if the volume ratio V1/V0 is set to the range between 2 to 20%.

As explained above, in this embodiment in which the tread rubber comprises functionally different compounds, the tire is effectively improved in conductivity, wear resistance, rolling resistance and wet properties.

Fig. 9 shows yet another embodiment of the present invention. In this embodiment, the tire 1 is also a radial tire for passenger cars with a relatively low aspect ratio.

In this example, the carcass 6 comprises a cord ply arranged radially at an angle of 75º to 90º with respect to the tire equator C and turned up around the bead cores in the bead portions to form a pair of turn-up portions 6b and a main portion 6a therebetween.

The belt 7 comprises two radially intersecting layers on the inside and outside, the cords of which are oriented at angles of 15º to 40º with respect to the tire equator C. In this example, steel cords are used as belt cords in order to provide good electrical conductivity.

In this embodiment, the conductive rubber compound 10 is partially used.

The tread rubber 9 comprises a base tread portion 10 and a cap tread portion 11.

The base tread portion 10 is arranged on the radially outer side of the belt 7 and is made of the conductive rubber compound 10 so that it has a volume resistivity of at most about 1 x 108 Ω cm. The width of the base tread portion 10 is substantially equal to the width of the belt 7. Its thickness is substantially constant over the entire width.

The cap tread portion 11 is arranged radially outward of the base tread portion so as to define most of the tread 2S. Its width is substantially equal to that of the base tread portion 10.

In order to reduce the rolling resistance, the cap tread portion 11 is made of the same compound as the main portion of the tread rubber in the second embodiment. That is, a compound mainly reinforced by silica is used. The carbon black content is preferably about 3 to 20 parts by weight based on 100 parts by weight of the diene rubber in order to meet the requirements for the cap tread portion 11 in terms of elasticity, hardness, heat generation and the like. If the carbon black content exceeds 20 parts by weight, the excellent effect of silica in low rolling resistance decreases, and therefore the rubber easily loses suppleness. As mentioned above, the upper limit of the silica content is 100 parts by weight. If it exceeds 100 parts by weight, it is difficult to limit the carbon black content to the preferred range, and deterioration due to photo-oxidation may occur, lowering the weather resistance. Thus, the cap tread portion 11 is a non-conductive rubber having a volume resistivity of over 1 x 108 Ω·cm.

Preferably, the ratio (h1/h2) of the thickness h1 of the cap tread portion 11 to the thickness h2 of the base tread portion 10 is set to the range between 1.5 and 4.0.

In order to conduct the electricity to the ground, a conductive section running continuously in the circumferential direction or a plurality of conductive sections 12 spaced apart in the circumferential direction are provided in the ground contact surface 2S.

The conductive portion 12 extends from the base tread portion 10 to the tread 2S and penetrates the cover tread portion 11.

The base tread section 10 as well as the conductive sections 12 are made of conductive rubber compound with a volume resistivity of at most 1 x 10⁸ Ω cm.

In Fig. 9, the conductive portion 12 is arranged along the tire equator C so that it can contact the ground with sufficient ground pressure during cornering as well as during straight running. However, it is also possible to form the conductive portion 12 at two or more axially spaced positions, for example on both sides of the tire equator C.

As an alternative to a conductive section 12 running continuously in the circumferential direction, this can be designed in the form of independent poles or columns. In a cross-section parallel to the tread, the cross-sectional shape can be designed as a circle, rectangle or the like. Such independent conductive sections can be arranged in a row or in rows in the circumferential direction. Furthermore, an isolated arrangement is also possible. In any case, the Arrangement should be such that one or more conductive portions always appear in the ground contact patch of the tire. Preferably, the corner between the base tread portion 10 and the conductive portion 12 is rounded at its radially inner end 12b to prevent stress concentration.

The axial width Wt of the conductive portion 12 at the outer end 12t or the tread 2S is preferably set in the range between 0.5 and 20.0 mm, or more preferably between 5 and 20 mm. If the width Wt is less than 0.5 mm, the electrical contact resistance to the road surface becomes unstable. If the width Wt exceeds 20 mm, the conductive portion 12 can reduce the rolling resistance and wet performance of the top tread portion 11.

In order to prevent cracks at the inner end 12b, the conductive portion 12 is provided radially outside the inner end 12b with a narrow-width portion 13 that is narrower than the inner end 12b. The minimum axial width in the narrow-width portion 13 is preferably in the range of 60 to 80% of the maximum width Wb at the inner end.

In Fig. 10, the width-narrow portion 13 is formed at the outer end 12t, with the width gradually decreasing from the inner end 12b to the outer end 12t, so that the width at the tread 25 becomes minimal.

Fig. 11 shows another example of the conductive portion 12, in which the width-narrow portion 13 is formed in the middle of the radial length of the conductive portion 12. The width decreases from the width shown in the Wide narrow portion 13 toward both the outer end 12t and the inner end 12b, the outer end 12t and the inner end 12b having substantially the same axial width.

Fig. 12 shows a cross section of a strip of raw tread rubber. The raw rubber compounds for the top tread portion, the base tread portion and the conductive portion were extruded from dies of an extruder and assembled into a body as raw tread rubber. Such a strip was split off in thin layers and wound around the circumference of the tire, with the ends spliced in the circumferential direction. Thus, in the tread portion of the tire, 90% or more conductive short fibers are oriented in the circumferential direction of the tire. As a result, the base tread portion 10 has a directional complex elastic modulus such that the complex elastic modulus E*c in the circumferential direction of the tire is not less than 1.1 times the complex elastic modulus E*a in the axial direction of the tire, whereby the rigidity in the circumferential direction of the tire can be increased without impairing the ride comfort.

In this example, the tread rubber further comprises wing rubbers 15 arranged at the axial ends of the cap tread portion 11 and the base tread portion 10.

The carbon black content of the conductive rubber compound is preferably 35 parts by weight or less based on 100 parts by weight of the rubber base. If the carbon black content exceeds 35 parts by weight, hysteresis loss tends to increase.

Furthermore, if the short fiber content is high enough to exert a satisfactory reinforcing effect on the conductive rubber compound, the carbon black content can be reduced substantially to zero, thus contributing to a further improvement in rolling resistance.

When silica is added together with the conductive short fibers to reduce the hysteresis loss, the silica content is preferably limited to 10 parts by weight or less. However, it is more preferable to reduce the silica content to zero.

In the conductive rubber compound and the non-conductive rubber compound, the carbon black is not limited to a specific type of carbon black, but carbon black whose particle diameter is 30 nm or less, i.e. hard carbon black, can be preferably used.

In this embodiment, the cap tread portion 11 has been reinforced by silica, while the base tread portion 10 has been reinforced by conductive short fibers and optionally by a minimum amount of carbon black. Therefore, the hysteresis losses of both rubbers 10 and 11 are small. In this case, by further reducing the loss factor of both the cap tread portion 11 and the base tread portion 10, the rolling resistance can be further reduced while maintaining good ride comfort. Relative reduction in the base tread portion 10 is preferable because it reduces heat generation on the inside of the tire during driving. However, a relative reduction in the cover tread section 11 is more preferable since this section contributes more effectively to reducing the rolling resistance than the base tread section 10. In this example, the loss factor of the base tread section 10 is therefore smaller than that of the cover tread section 11.

In this embodiment, the wide conductive base tread portion is disposed at the radially inner end of the conductive portion and extends along the belt, with at least the axial edges of the base tread portion being electrically connected to the sidewall rubber and the bead rubber. Accordingly, it is not always necessary to use a steel belt.

Comparison test 3

For testing purposes, pneumatic radial tires (size: 205/65R15) having the structure shown in Fig. 9 were manufactured using various tread rubber compounds shown in Table 3 and tested for rolling resistance, electrical resistivity and wet properties. The test results are shown in Table 4.

In the test tires, the bead portions and the sidewall portions were made of a rubber compound with a volume resistivity of about 1 x 107 Ω cm. The base tread portions 5 to 8 had the same ratio of the complex elastic modulus (E*c/E*a) of 1.1.

Note that the complex elastic modulus E* and the loss coefficient in the present invention were measured by a viscoelastic spectrometer under the following conditions: temperature 70ºC, initial strain 10%, dynamic deformation ±1%, frequency 10 Hz. Table 3

*1) logarithmic indication

S-SBR: Styrene 15%, Vinyl 57%, non-oil extended

BR: ice 98%

Silica: BET = 175 m²/g, DBP oil absorption = 210 ml/100 g

Soot A: primary particle diameter 16 nm

Soot B: primary particle diameter 28 nm

Conductive fibers 1: L = 800 um, D = 16 um

Conductive fibers 2: L = 400-600 um, D = 16 um Table 4

Claims (17)

1. A tire comprising a tread rubber (2) whose radially outer surface forms the ground contact surface (25) of the tire, the tread rubber at least partially containing a conductive rubber extending from the radially inner surface of the tread rubber to the ground contact surface, characterized in that the conductive rubber is mixed from 100 parts by weight of diene rubber and 2 to 30 parts by weight of conductive short fibers formed by coating short reinforcing fibers with a conductive substance, and that the conductive rubber has a volume resistivity of at most 1 x 10⁸ Ω cm. 2. Tire according to claim 1, characterized in that the length of the conductive short fibers is 10 to 6000 µm and the length/diameter ratio of the conductive short fibers is 10 to 2000. 3. Tire according to claim 1 or 2, characterized in that the short reinforcing fibers are organic fibers and the conductive substance corresponds to one or more polymers whose main chain has a pi-electron conjugation. 4. Tire according to claim 3, characterized in that the short reinforcing fibers are nylon fibers or cellulose fibers, the conductive substance being a mixture in which each polymer has a main chain consisting of pyrrole rings or aniline rings. 5. Tire according to claim 4, characterized in that the conductive substance contains a small amount of an electron-accepting substance such as iodine, arsenic (V) fluoride and the like or an electron-donating substance such as potassium, sodium and the like. 6. Tire according to claim 1 or 2, characterized in that the short reinforcing fibers are organic fibers and the conductive substance is a metal salt. 7. Tire according to one of claims 1-6, characterized in that substantially all of the tread rubber is a conductive rubber. 8. Tire according to one of claims 1-6, characterized in that the tread rubber further comprises a less conductive rubber, the conductive rubber forming a circumferentially extending conductive portion of the ground contact surface, and the less conductive rubber forming the remaining main portion of the ground contact surface. 9. Tire according to one of claims 1-6, characterized in that the tread rubber further comprises a less conductive rubber, the conductive rubber forming a plurality of circumferentially spaced conductive portions of the ground contact surface, and the less conductive rubber forming the remaining major portion of the ground contact surface. 10. Tire according to one of claims 1-6, characterized in that the tread rubber further comprises a less conductive rubber, the conductive rubber forming a circumferentially extending conductive portion and a plurality of circumferentially spaced conductive portions of the ground contact surface, and the less conductive rubber forming the remaining major portion of the ground contact surface. 11. Tire according to one of claims 8-10, characterized in that a steel cord belt (7) is arranged immediately inside the radially inner surface of the tread rubber (2), the volume of the conductive rubber being 2 to 20% of the total volume of the tread rubber and the maximum/minimum ratio of the axial width of the conductive rubber being 1 to about 5. 12. Tire according to claim 11, characterized in that the axial width of the conductive rubber decreases from the radially inner end outwards. 13. Tire according to one of claims 8-10, characterized in that the conductive rubber forms a base tread portion (10) which defines the radially inner surface of the tread rubber and on which the less conductive rubber is provided, and that one or more ground contact portions (13) extend radially outward from the base tread portion to the ground contact surface. 14. Tire according to claim 13, characterized in that the one or more ground contact portions (13) on the radially outer other side of its radially inner end and have a narrow section in width. 15. A tire according to claim 13 or 14, characterized in that the less conductive rubber is mixed from 100 parts by weight of diene rubber, 30 to 100 parts by weight of silica and 3 to 20 parts by weight of carbon black, and its volume resistivity is not less than 1· 10⁸ Ωcm. 16. A tire according to claim 13, 14 or 15, characterized in that the loss coefficient of the conductive rubber in the base tread portion is less than the loss coefficient of the less conductive rubber thereon. 17. Tire according to claim 13, 14, 15 or 16, characterized in that the base tread portion has a direction-dependent complex elastic modulus such that the complex elastic modulus E*c in the circumferential direction of the tire is not less than 1.1 times the complex elastic modulus E*a in the axial direction of the tire.